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We studied the thermal tolerance of two populations of the tree lizard Urosaurus ornatus from a desert environment in southwestern New Mexico and a low montane environment in southeastern Arizona. In general, there were no differences in thermal tolerance between the two populations. Desert individuals tended to have higher panting thresholds in the summer than low montane individuals, whereas the reverse held for late summer and fall. Males had lower critical thermal maxima than females in the low montane population, but there was no difference in the desert population. Thermal tolerance varied from month to month, tolerances being highest during the summer. Thermal tolerance was related to body size (measured as both snout-vent length and body mass), explaining up to 39% of the variance.